INFORMATION wants to be free. This decades-old slogan is the philosophical heart of the internet, putting nearly all human knowledge at our fingertips, free to anyone with a connection.

Here is another old slogan: if you’re not paying, you’re the product. We might not hand over cash for many of the services we get from the internet giants, but we do pay in cold, hard data. On the whole, we have been happy to make that pact. But as the row over Facebook data gathered by Cambridge Analytica shows, many are starting …